1. Field of the Invention
The present invention relates to a method for molding a vehicular wheel rim made of a metal plate or protruded member.
2. Related Art
Japanese Patent Laid-Open No. 2005-95982 Gazette (US2005091850A1) describes a method for manufacturing a wheel for use in passenger cars, trucks, buggies and the like. It then discloses a method for molding a wheel rim by disposing an electromagnetic molding die whose inner face serves as a molding face at an outer peripheral side of a cylindrical material made of aluminum alloy, disposing an electromagnetic molding coil at an inner peripheral side of the cylindrical material, charging electrical energy to the electromagnetic molding coil in this state to enlarge and press the cylindrical material against the molding face of the molding die to mold a cylindrical body having a drop portion whose outer diameter is small and both side portions whose outer diameter is larger than that of the drop portion and by curling edges of the side portions.
It is noted that the electromagnetic molding refers to a technology of molding a workpiece into a predetermined shape by making use of a phenomenon in which an electromagnetic molding coil produces a strong magnetic field in a very short time when electric energy (electric charge) accumulated at high voltage is instantaneously charged (discharged) into the electromagnetic molding coil and the workpiece placed in the magnetic field is subjected to a strong enlarging force or contraction force caused by a repulsive force (Lorentz force according to Fleming's left-hand rule) of the magnetic field and thereby undergoes fast plastic deformation. In case of this example, the cylindrical material enlarges in a direction of the outer diameter by receiving the strong enlarging force and is pressed against the molding face of the electromagnetic molding die.
Because an extruded member having an equal thickness or a plate member in a cylindrical shape formed by bending and welding edge portions thereof is used as the cylindrical material and the side portions are enlarged more than the drop portion, the above-mentioned method allows the wheel rim having the thicker drop portion (thickness decreases less in enlarging the diameter thereof) and the thinner both side portions (thickness decreases more in enlarging the diameter) to be molded. Therefore, when a disk is fixed to the drop portion, the wheel rim hardly causes welding distortion and can support a large load corresponding to its thickness. Still more, if the thickness of the drop portion is supposed to be equal, weight of the entire wheel rim may be lightened due to the thin both side portions. It should be noted that Japanese Patent Laid-Open No. 2004-224292 Gazette also describes a curling of edges of the wheel rim.
However, there has been a problem in pressing a curling die against the edges of the side portions to bend and curl the edges in an external direction (external curling process) in a curling process after the electromagnetic molding that the drop portion of the cylindrical body buckles due to the pressure in the axial direction of the curling die.
Accordingly, it is an object of the invention to solve the above-mentioned problem in molding the wheel rim from the cylindrical material by applying the electromagnetic molding method and the curling method by pressing.